The present invention relates to a novel improved ammonium uranate and processes for its preparation, and more particularly, this invention provides an ammonium uranate in the form of substantially spherical particles with a well-controlled particle size, an increased apparent density and very good flowability, together with a process for obtaining such an ammonium uranate by crystallization and agglomeration of uraniferous feed solutions.
Ammonium uranate is the uraniferous concentrate customarily used in plants for the conversion of uraniferous concentrates to the fluorides or oxides of uranium required in the fabrication of nuclear fuel elements. According to the actual extraction technique described, for example, by R. Merrit, The Extraction Metallurgy of Uranium, Library of Congress Catalog, at pages 240-246, the uranium contained in uraniferous minerals is recovered after dissolution of the mineral, purification, and concentration of the uraniferous solution, in the form of acid or alkaline liquors. The uraniferous concentrates are obtained by precipitation starting with acidic or basic solutions in a medium of sulfate SO.sub.4.sup.--, and/or carbonate, CO.sub.3.sup.--, ions and particularly with ammonia.
The precipitation is carried out in a series of vessels with agitation and a temperature of 60.degree.-70.degree. C. The pH is adjusted between 7 and 8 to a value which increases slowly from one vessel to another. From this process there results a very fine ammonium uranate precipitate (the finer as the pH is raised) having a thixotropic character which is difficult to filter and dry.
These particles have an irregular form, and the product obtained possesses poor flowability. The handling of such a product causes health and safety problems due to the discharge of dusts rich in uranium, as in plants for the preparation of uranium-bearing concentrates, during drying and loading of the solid into casks, and as in refineries which recover these uranium-bearing concentrates for purification and conversion to oxide, fluoride and metal.
The ammonium uranate should moreover have a sulfate ion, SO.sub.4.sup.--, content below 3% (Allied Co standard) but the tendency is toward a lower limit of 2%. These SO.sub.4.sup.-- ions arise from the formation of basic uranyl sulfate according to the equation:
(1) 2UO.sub.2 SO.sub.4 +2NH.sub.4 OH+4H.sub.2 O .fwdarw.(UO.sub.2).sub.2 SO.sub.4 (OH).sub.2.4H.sub.2 O+(NH.sub.4).sub.2 SO.sub.4 PA1 (2) (UO.sub.2).sub.2 SO.sub.4 (OH.sub.2.4H.sub.2 O+4NH.sub.4 OH .fwdarw.(NH.sub.4).sub.2 U.sub.2 O.sub.7 +(NH.sub.4).sub.2 SO.sub.4 +7H.sub.2 O
which in the presence of an excess of NH.sub.4 OH leads to ammonium uranate according to the equation:
following the Merrit reference cited above.
The requirement to obtain a low content of SO.sub.4 ions leads to carrying out the precipitation of the ammonium uranate at a more alkaline pH, thereby causing a more abundant nucleation resulting in a pulverulent product which is dangerous to handle.
These is accordingly a commercial need to handle ammonium uranate in a manner which will give greater handling safety, that is, with a greatly reduced discharge of uraniumrich powders, and at the same time providing good purity, particularly with respect to SO.sub.4 ions.